Steam reforming reaction is commonly used as a method of generating hydrogen to be supplied to a fuel cell. In the steam reforming reaction, for example, a reformer including a Ni (nickel)-based or Ru (ruthenium)-based reforming catalyst is used. In the reformer, a raw material (e.g., city gas supplied through piping in a city area or LP gas) is reacted with steam at a high temperature of approximately 600° C. to 700° C., and thereby a hydrogen-containing gas containing hydrogen as a main component is generated.
At the time, heating the reformer is necessary for causing the steam reforming reaction to progress, and a heating method commonly used for heating the reformer is as follows: a fuel off-gas, which remains after a fuel gas is used in electric power generation by a fuel cell, is combusted by a combustor (e.g., a burner). Here, water supplied to the reformer is evaporated by using an evaporator, and thereby steam for use in the reforming reaction of the reformer is generated.
The hydrogen-containing gas generated in the reformer contains carbon monoxide. The carbon monoxide poisons catalysts included in the fuel cell, and thereby hinders the electric power generation. Therefore, it is common that a shift converter configured to cause a shift reaction and a selective oxidizer configured to cause a selective oxidation reaction are provided for the purpose of reducing the concentration of carbon monoxide in the hydrogen-containing gas generated in the reformer. Accordingly, the reformer, shift converter, and selective oxidizer in a fuel cell system may be collectively referred to as a hydrogen generation apparatus.
Generally speaking, the raw material supplied to the reformer contains sulfur compounds. Specifically, city gas and LP gas each contain a sulfur content derived from their raw material, and also, sulfur compounds such as sulfides and mercaptans are added to these gases as odorants for the purpose of gas leakage detection.
It is known that such sulfur compounds negatively affect the reforming reaction, that is, the sulfur compounds poison and degrade the Ni-based and Ru-based reforming catalysts commonly used in the steam reforming reaction.
Therefore, the raw material such as city gas or LP gas is subjected to a suitable desulfurization process before the raw material is supplied to the hydrogen generation apparatus. In general, the raw material is desulfurized by a method in which zeolite (an adsorbent) is used to remove sulfur compounds from the raw material through normal-temperature adsorption.
However, in such a normal-temperature desulfurization method, it is necessary to replace the adsorbent every predetermined period since the amount of sulfur removed through the adsorption by the method is small. Accordingly, in a case where the hydrogen generation apparatus is operated for a long term, there is a disadvantage of high maintenance costs.
In order to overcome such a disadvantage, hydrodesulfurization methods with which sulfur can be removed by a large amount have been developed. For example, in one hydrodesulfurization method, a hydrogenation catalyst is used to react the sulfur compounds contained in the raw material with hydrogen at approximately 200° C. to 400° C., so that the sulfur compounds are transformed into hydrogen sulfide, and thereafter, an adsorption catalyst is used to adsorb the hydrogen sulfide at approximately 200° C. to 350° C. In this manner, the sulfur content in the raw material can be properly removed.
At the time of stopping the operation of the hydrogen generation apparatus, input and output portions that serve to allow the inside of the reformer to be in communication with the outside of the reformer are sealed so that combustible gases such as the raw material and the hydrogen-containing gas will not leak to the atmosphere (to the outside). Sealing the input and output portions also prevents external air from entering the inside of the reformer. The reforming catalyst provided in the reformer degrades if the catalyst is exposed to an oxidation gas (air) at a high temperature. Therefore, it is important to prevent external air from entering the inside of the reformer.
However, if the input and output portions are kept sealed, the inside of the reformer may become excessively pressurized or the inside of the reformer may become an excessive negative pressure state. If the inside of the reformer has become excessively pressurized, a solenoid valve for use in the sealing, or the like, is opened and closed, so that the pressure in the reformer is temporarily released to the atmosphere and thereby the inside of the reformer is depressurized. If the inside of the reformer has become an excessive negative pressure state, the raw material in a predetermined amount is forcibly supplied into the reformer. In this manner, the inside of the reformer is pressurized. These depressurizing operation and pressurizing operation are hereinafter referred to as pressure keeping operations of the reformer. By performing these pressure keeping operations, the operation of the hydrogen generation apparatus can be stopped properly with the internal pressure of the reformer kept in a suitable state, so that loads are not put on component devices.
However, if the operation of the hydrogen generation apparatus is stopped for the reason that electric power supply is cut off during the operation of the hydrogen generation apparatus due to power outage or the like, then the above-described pressure keeping operations cannot be performed. Accordingly, the inside of the reformer is left sealed, which may result in that the inside of the reformer becomes excessively pressurized due to evaporation of water remaining in at least one of the evaporator and the reformer.
In this respect, there is proposed a hydrogen generation apparatus including a depressurizer capable of depressurizing the inside of the reformer even when electric power supply is cut off (see Patent Literature 1).